Killed factories may kill a completed buildee

Factories kill their curBuild when killed themselves:
https://github.com/beyond-all-reason/RecoilEngine/blob/6a5f3278e7e02a91d3dca76ee353584656871773/rts/Sim/Units/UnitTypes/Factory.cpp#L69-L78

But it seems possible that an assisting builder may have completed the buildee already. The cost of checking whether the unit is completed seems worth closing off the possibility.

This is subframe nonsense and not a pressing issue to gameplay.

Start in rts/Sim/Units/UnitTypes/Factory.cpp at lines 69-78, where a factory kills its current buildee when the factory is killed. Trace the available unit-completion state and verify the cleanup only affects an incomplete buildee. Done means a completed buildee is left alive while an incomplete one still follows the existing behavior.

Written by the indexing model from the issue text.